Well, in addition to your own cells happily doing their jobs, you are hosting many tiny organisms. Many, that’s really trillions. And very tiny, they’re microscopic.
Thinking “yuk”… just hang on. These microbes are really good tenants. Like the guests that clean your house, take out the garbage, and mow the lawn without being asked. They even get along, help each other and discourage bad behaviour. You really don’t want them to leave.